Hybrid vehicles are typically driven by two primary power sources, e.g., an internal combustion engine and an electric motor (powered by a battery). One type of hybrid vehicle is a power-split hybrid. A power-split hybrid allows the engine and the electric motor to separately, or in combination, supply power to the wheels. A planetary gear set can be coupled to the engine and to a generator such that the engine can charge the battery even while the electric motor supplies the necessary torque to propel the vehicle. The generator may also act as an electric motor by supplying torque through the planetary gear set. One of the planetary gears (e.g., the ring gear) can also couple the engine and the generator to an output, such as a differential that distributes torque to the wheels.
Particularly at low vehicle speeds, a situation may arise in which the amount of wheel power may be very small even when the wheel torque demand is relatively large. The most extreme of such a scenario is when the vehicle is launching from rest at full accelerator pedal where the wheel speed is zero (and hence the wheel power is also zero), yet the wheel torque demand is at a maximum. Another similar situation may arise when the vehicle is at low or zero speed while on sand or gravel, and the vehicle operator sharply depresses the accelerator pedal. In these and other similar scenarios, with a normal battery state of charge (SOC) the resulting desired engine power corresponds with that which is necessary to overcome losses in the transmission and accessory loads.
Hybrid vehicle architecture should be designed to properly accommodate such driving scenarios while providing torque to the wheels to propel the vehicle as expected. In one type of power-split hybrid, a series of reduction gears transfer torque from the planetary gear set to a differential which distributes the torque among the wheels. For example, in one vehicle, an engine may be connected to a carrier of a planetary gear set, while the sun gear is connected to a generator. Torque output by the engine causes rotation of the sun gear, which turns the generator to either charge a battery or provide drive torque through a traction motor. Additionally, a ring gear of the planetary gear set turns a series of torque reduction gears, ultimately leading to the fraction motor and to an output of the powertrain. Control strategies exist that work to deliver optimum torque to the wheels during the low-speed and high-torque scenarios described above based on the amount of torque transmitted through the ring gear. However, these control strategies account for the series of reduction gears when deciding how much torque to produce from the engine and/or the electric motor.
In another type of power-split hybrid, a chain, belt, or other continuous torque transfer member transfers the torque from the planetary gear set to a differential. No gear reduction is provided.